class Solution {

    int preIndex = 0;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return buildTreeChild(preorder, inorder, 0, inorder.length-1);
    }

    public TreeNode buildTreeChild(int[] preorder, int[] inorder, int inbegin, int inend) {
        if(inbegin > inend) {
            return null;
        }

        TreeNode root = new TreeNode(preorder[preIndex]);
        int rootIndex = findIndex(inorder, inbegin, inend, preorder[preIndex]);
        preIndex++;

        root.left = buildTreeChild(preorder, inorder, inbegin, rootIndex-1);
        root.right = buildTreeChild(preorder, inorder, rootIndex+1, inend);

        return root;
    }

    public int findIndex(int[] inorder, int inbegin, int inend, int key) {
        for(int i = 0; i <= inend; i++) {
            if(inorder[i] == key) {
                return i;
            }
        }
        return -1;
    }

}